Signal generator with supply voltage compensation



P. H. LUFT Filed NOV. 14, 1956 @Q N gbm QSR Q March 8, 1960 SIGNALGENERATOR WITH SUPPLY VOLTAGE COMPENSATION INVENTOR. P120190 H Laf. BY

A 150.5 HTIOQMEY United states-Pt t m SIGNAL GENERATOR Wl T I-I'SUPPLYVOLTAGE I COMPENSATION Application November 14, 1956, Serial No. 622,123

' 2 Claims. c1. 330-29 My invention relates to a signal generatorwithsupply voltage compensation and more particularly to a transistorizedamplifier in which small variations in the operating voltage arecompensated for so as to have little effect upon the output'si gnalvoltage.

In a signal generator in which a signal developed by an oscillator is;to be amplified by anamplifier and in which the oscillator and amplifierare provided with operating voltages from a common source, the amplifiedsignal usually varies in accordance with variations of the supply'voltagei For example, if the operating voltages are supplied by' abattery, the amplified signal decreases as the battery ages andincreases when an old battery is replaced with a new one. In someapplications of signal generators of the type embodying my invention itis essential that the output signal be maintained substantially constantand independent of such supply voltage changes. An application of thistype occurs in railway signaling wherein a track circuit is formed, forthe purpose -of detecting when a vehicle is Within certain sections oftrack, by connecting a signal generator or transmitter and a receiver tothe track rails. energizes the track rails with a current which flowsalong the track rails to the receiver. The receiver controls apparatusforiindicating the persence or absence of a ve hicle within the tracksection. When a 'vehicle is located a distance from the signal generatorand receiver, the track rails present a high impedance to the signalcurrent and prevent it from being shunted between the rails due to theshunt formed bythe vehicles wheels and axles and, accordingly, a clearindication is provided. However, when the vehicle approaches to withinsome predetermined distance of the signal generator and receivingapparatus, the signal is shunted through the shunt formed by the wheelsand axles of the vehicle, and

the apparatus controlled by the receiver responds accordingly toindicate the presence of the vehicle within the track section.Obviously, if the output. from the signal generator varies from time totime, so would the distance I 'to the point at which the vehiclespresence would be recognized. Since variations in the voltage of thesupply source for such a signal generator causes variations in theoutput signal voltage, it is desirable to provide the signal generatorwith means for compensating for variations in the supply voltage.

It is therefore, an object of my invention to provide a signal generatorwith an output signal that is substantially independent of smallvariations of the operating .voltage. j f

A further object ofmy invention is to provide a signal generatorincluding a transistorized amplifier having The signal generator takenin connection with the accompanying drawing and the new and novelfeatures thereof will be set forth in the appended claims.

The accompanying drawing is a diagrammatic view showing a signalgenerator provided with means embody ing my invention for obtaining aconstant output voltage.

The embodiment of thegenerator illustrated in the drawing comprises anoscillator 1 which generates a signal voltage to be amplified byamplifier 2. The amplified signal may, as here shown, he furtheramplified by a power amplifier 3. The useful output signal from thesignal generator is obtainable at the output circuit of the amplifier,here designated vby the reference character 4. The oscillator andamplifier are supplied with suitable operating voltages from a commonsource of direct current energy, here shown as a battery 5.

AmplifierZ is a variable gain transistorizedamplifier comprising aninput transformer 6 which is provided with a primary winding 7 and asecondary winding 8. The primary winding is connected to the oscillator1 in such manner that the transformer is energized by the signalvoltage. The secondary winding is connected to a voltage dividingnetwork here shown as series connected resistors 9 and 10. The commonjunction 11 of these two resistors is further connected to an emitterelectrode 12,.of a transistor 13. The end of the secondary winding 8which is connected to resistor 10 is connected through a pair of seriesconnected resistors 14 and 15 to a base electrode 16 of the transistor.Resistors 14- and 15 are by-passed by a capacitor 17 which has a lowreactance at the frequency of the signaling current. The common junctionof resistors 14 and 15 is connected to the positive terminal of battery5. Collector electrode 18 of the transistor is connected through theprimary winding 19 of a transformer 20 and a resistor 21, connected inseries, to the base electrode. A

' common junction 22 between the resistor and primary winding isconnected to the negative terminal of battery 5. Secondary winding 23 ofthe transformer 20 is tuate with variations of the voltage of battery 5if supply voltage compensation were not provided. However, with my novelarrangement the rectifying characteristics of a transistor element areemployed to compensate for such variations of the supply voltage. basecircuit of the transistor associated with amplifier 2 is arranged sothat the gain of amplifier 2 is made to vary inversely as the voltage ofbattery 5. That is, an increase in the supply voltage results in adecrease in gain and vice versa. 1

Before proceeding with the description of the mode of operation ofamplifier 2 it may be well to review briefly someof the internalcharacteristics of a transistor which are utilized in my invention toenable a better understanding of my invention. It is well known to thoseskilled in the art that the internal characteristics of a transistorsuch, for example, as the emitter to base circuit characteristicsbehave, insofar as an external circuit connected thereto is concerned,as a simple rectifying I contact. The volt ampere characteristics ofsuch a recti- The emitter to fying contact are highly non-linear.'Inasmuch' as the incremental resistance of such a rectifying contact isde-.

termined by the slope of the volt ampere characteristics at the point towhich the emitter to base circuit is biased, the incremental resistancevaries over a considerable range of values for various values of biasingvoltage. When the emitter to base circuit, referred to hereinafter as arectifying junction, is forward biased, the incremental resistance isrelatively low and decreases as the bias voltage is increased within thenormal range of bias voltages. When a junction is reverse biased, theincremental resistance is relatively high and ordinarily, remains morenearly constant than the forward biased junction, within reasonablelimits of reverse bias voltage. It is the variation in the incrementalresistance of a forward biased junction that is utilized in my inventionto compensate for the variations of the supply voltage.

For the purpose of understanding the operation of my invention let it beassumed that the circuit is operating normally with a given batteryvoltage. Accordingly, the transistor is provided with proper operatingvoltages due to the voltages developed across resistors and 21. Thevoltage drop across resistor 15 provides the operating voltage for theemitter to base circuit of transistor 13. This voltage biases thiscircuit in the forward direction, and the voltage developed acrossresistor 21 provides the operating voltage for the collector to basecircuit of the transistor. 7

The oscillator provides the secondary winding 8 of transformer 6 with asignal voltage, a part of which is impressed upon the emitter to basecircuit, of the transistor. The part of the secondary voltage which issupplied to the emitter to base circuit is determined by the volt agedividing network comprising resistor 9 acting in series with theparallel combination of resistor it and the internal incrementalresistance of the emitter to base circuit' of the transistor (theimpedance of capacitor 17 being negligible at the frequency of thesignal current). This establishes a definite emitter to base signalvoltage and, in accordance therewith, a signal voltage is developed inthe collector to base circuit. This signal voltage passes throughtransformer 26 to the power amplifier 3 which, in turn, provides theoutput circuit with a signal voltage.

If the battery voltage decreases due, for example, to aging of thebattery, the oscillator provides a signal voltage with decreasedamplitude which would result in a decreased signal at the output circuitof amplifier 3 if compensation was not provided. Also amplifier 3 mayhave reduced gain with the lower operating voltage which would alsodecrease 'the output signal voltage. The decrease in supply voltage,however, decreases the forward bias on the emitter to base circuit oftransistor 13 which results in an increased emitter to base incrementalresistance. The increased incremental resistance changes the voltagedividing action of resistors 9 and it) and the emitter to base circuitof the transistor. causes a greater portion of the signal voltageinduced in the secondary winding 8 of transformer 6 to appear across theemitter to base circuit of the transistor. The increased input signalvoltage to the emitter to base circuit of the transistor results in agreater amplified signal in the collector to base circuit. Thus byproper proportioning of parts, the voltage developed at the outputcircuit of amplifier 3 can be maintained substantially constant.

An increase in the supply voltage such as would occur when the batteryis replaced by a new battery has the opposite effect. is, an increase inthe battery voltage provides a greater biasvvoltage on the emitter tobase circuit of the transistor, due to the increased voltage acrossresistor 15. The increased bias voltage lowers the emitter to baseincremental resistance of the transistor and a smaller portion of thesignal voltage induced in the secondary winding 8 appears across theemitter to This . 4 V 7 base circuit which compensates for the increasedsignal voltage developed by the oscillator, V v V i It is to beunderstood that the current gain of the transistor (usually referredto'in the literature as alpha) increases with an increase in the batteryvoltage due to the increased supply voltage to the collector to basecircuit and decreases with decreased supply voltage. These changes incurrent gain, however, are more than cornpensated for by the voltagedividing action of resistors 9 and 1S and the incremental resistance ofthe emitter to base voltage. Accordingly, it is the overall gain of theamplifier stage represented by the reference character 2 whichcompensates for the variations of the battery voltage.

It should be clear that while the drawing shows the use of a transistorof the PNP variety in the compensating amplifier, that a transistor ofthe N-PN variety may, equally as well, be used by'merely reversing thepolarity of the operating voltage supply; Furthermore, while the drawingshows the use of a transistor in a circuit arrangement that is usuallyreferred to as a common base circuit, other circuit arrangements such ascommon emitter or common collector circuit may also be employed.

Although I have herein shown and described only one form of supplyvoltage compensating means embodying my invention, it is to beunderstood that various changes and modifications may be made thereinwithin the scope of the appended claims without departing from thespirit and scope of my. invention.

Having thus described my invention, what I claim is:

1. A compensating circuit for providing a substantially constantamplitude signal comprising an oscillator generating a signal voltage, atransistor including an emitter, a base, and a collector electrodeconnected to amplify said signal voltage, said oscillator and said transistor adapted to be connected to a common battery for obtaining properoperating and biasing voltages therefrom, the battery being susceptibleto voltage variations, the amplitude of said signal output from saidoscillator being proportional to the battery voltage, a voltage dividingnetwork comprising an impedance having at least first and secondportions connected in series and the emitter to base circuit of saidtransistor'connected in parallelrwith said second portion, theincremental impedance in said emitter to base circuit varying inverselywith the biasing voltage derived from the battery, and means impressingthe generated signal across said network, said network effecting avoltage division to cause the signal impressed across said emitter tobase circuit to be inversely related to the amplitude of the generatedsignal thereby to compensate for variations in the ampli tude of thegenerated signals due to variations in the battery voltage. a r

2. A compensating circuit for providing a substantially constantamplitude signal comprising means generating a signal voltage, atransistor including an emitter, a base, and collector electrodes, saidsignal generating means and said transistor being adapted to beconnected to a common source of direct current energy for obtainingproper operating voltages therefrom, the direct current energy sourcebeing susceptible to voltage variations, the amplitude of said signaloutput from said signal generating means being proportional to thevoltage of said source of energy, a voltage dividing network includingat least first and second impedances connected in series and the emitterto base path of said transistor connected in parallel to said secondimpedance, and means impressing said input signal across said network,the incremental impedance existing between said emitter tosaid basebeing inversely related to the voltage of the source of energy, anyvariation in said incremental impedance due to variations in the sourcevoltage causing achange in the voltage dividing action of said networkwhereby the signal voltage effectively impressed across ,;a,o2e,oe0 6FOREIGN PATENTS Australia Apr. 20, 1939 OTHER REFERENCESPublication-Radio and Television News-.March 1954, SuperregenerativeTransistor Receiver? by Joseph said emitter to base varies inverselywith the change in voltage of the source of energy. 107,119

References Clted in the file of this patent 6 UNITED STATES PATENTS M.Hughes, pp. 72, 73. 1 755 865 Butler 7 Apr 22 1930PublicationElectronies-May 1955, page 175, Transistor Amplifier." j w, 719512148 1934 10 Publication-Electronics-July 1955--pp. 119-1212,570,939 Goodrich Oct. 9, 1951 Transistor Amplifiers for AnalogComputers, by G. M. 2,751,550 Chase June1 9, 1956 Ettinger.

